Batteries for Electric Propulsion

1.0 Introduction

Which battery to choose? The batteries form one of the most expensive components in electric propulsion, but not because of their complexity. With batteries, you simply need a LOT of them. This becomes a serious challenge for space and weight, since batteries are also very heavy. Further, the market appears very complex, with dozens of battery vendors, all claiming they are the best. Today, I will review the basic battery chemistries out there, pros and cons, and best use scenarios for each chemistry. Armed with this knowledge, you can quickly determine which battery type best suits you, eliminating 90% of the marketing hype.

2.0 Batteries

When selecting batteries, we start by considering the chemistry. Which type of battery do you want? This depends on several factors:

Battery capacity
Depth of discharge / useable capacity
Weight
Volume / space for storage
Lifetime charge cycles
Cost

When comparing battery types, we often see charts like Figure 3‑1. But did you notice the little asterix that said “Based on Bare Cell”? These numbers come from a test battery; something only ever used in a lab. It doesn’t consider the weight of the casing, spacing of multiple cells, any included electronics to protect the battery. And it ignores cost. Is this actually realistic?

I prefer to compare real-world batteries in their final state. (Figure 2‑2) This offers a skewed perspective, because I only compare individual manufacturers, with no guarantee that I picked the best manufacturer in each category. But it provides an excellent overview to decide which battery chemistry warrants further scrutiny.

Figure 2‑2 shows the same comparison of weight and volume, but based on real world batteries. We see that they largely segregate into three categories. Lead acid and AGM batteries form the entry level. Nothing impressive from a weight and volume perspective. Lithium iron phosphate (LiFePO₄) stands out as the clear winner, both weighing less and requiring less volume. Then carbon foam stands in its own category. This doesn’t win any prizes for weight, with similar specific energy to lead acid. But it leads the pack in terms of volumetric density. Carbon foam packs the energy into a small space, requiring even less volume than LiFePO₄. Not a good choice for racing boats that need to minimize weight. But if you have a cruising boat and just need to fit lots of battery into a small space, carbon foam offers a good option. Which battery works you? Your lifestyle determines the best battery chemistry.

By lifestyle, I actually mean your wallet. It’s no secret that cost factors largely into purchase decisions. Better performance requires higher cost. So how much do we pay for those fancy lightweight batteries? Figure 2‑3 compares cost against specific weight. This combines the energy supplied, weight of the battery, and cost for purchase into one comparison. In this comparison, lithium batteries only cost about 20% more than AGM batteries. Sure, you pay more for lithium, but you get more as well. Looking at the graph, we also see that carbon foam batteries don’t win on a weight basis. The previous graph confirmed this: carbon foam batteries stand out for more energy in a smaller package. But there’s more to a battery than weight. We also care about the cost of the power they deliver.

With large investments like batteries, we especially need to ensure we get good value for our money. We want batteries that are good at storing electricity, for a reasonable price. I compared purchase price against energy storage in the batteries. Now things get interesting; value sways heavily based on two major factors:

Depth of discharge: To maintain battery life, you shouldn’t discharge a battery completely before recharging. This sets the useable capacity of your battery.
Total lifetime cycles: how many times can you recharge the battery before it stops working.

The answer shows a strong relationship between these two factors, leading to many complicated graphs and tables. I kept things simple by selecting the best values in each category, creating a best case comparison. But best value depends on how you use the battery.

Figure 2‑4 shows two cost comparisons. The Short Term Cost compares purchase price against the useable energy from a single charge of the battery. The Long Term Cost compares against the total energy stored in the battery across the lifetime of all recharge cycles. In the long term, LiFePO₄ batteries trounce lead acid. Lithium extracts a heavy price, but it may be the last battery you ever buy. Carbon foam batteries fall in very close to lithium. Less performance, but less cost as well. Here’s the catch. These high costs only make sense if you plan to use the full life of the battery.

Even with daily use, a lithium battery lasts around 16 years. Do you plan to use your boat for that long? The average weekend sailor barely uses their battery. They fall more into the short term comparison. In the short term, lead acid devastates lithium and carbon foam. Simple and cheap is the way to go for short term usage. Sure, it doesn’t win any performance competitions. But you never needed performance.

Performance or bargain basement? The best value for battery chemistry depends on your lifetime. How do you plan to use the batteries and how long will you own them?

3.0 Lithium Battery Safety

Given the high energy requirements for electric propulsion, many people opt for the performance of lithium ion batteries. But within the category of lithium ion, you run into several more choices. There are many different types of lithium ion chemistries. The chemistry favored by the recreational market is lithium iron phosphate (LiFePO₄). We like these for a safety reason: they do not suffer from thermal runaway as easily as the other batteries.

Thermal runaway. That’s a nice way to say that if your lithium battery gets too hot, it can ignite and burn a hole through your boat! More technically, all lithium chemistries have a temperature where the different chemistries start to break down, releasing oxygen and generating extensive heat. Thermal runaway happens when this starts in one cell of the battery, and the heat raises the temperature high enough to kick off the next cell. And the next and the next, until the entire battery is one hot ball of flame.

With lithium iron phosphate (LiFePO₄), this is extremely difficult. [2] [3] [4] Certainly, LiFePO₄ still heats and vents profusely, if used improperly. Lots of smoke, but rarely any fire. By design, the heat of a single cell should not raise the temperature high enough to ignite the neighboring cells. One major exception is a puncture to the battery. Some type of physical damage that creates a new conductor between cells and allows rapid current flow. This could result in major heat and may possibly create thermal runaway in a few circumstances. But I place that as a low risk. Ask yourself, what scenario results in a physical puncture to the protected battery? Almost any answer to that starts with some other disaster that will likely take precedent. For all but a few unlikely events, thermal runaway doesn’t happen with LiFePO₄ batteries. When shopping for marine lithium batteries, LiFePO₄ is the only safe choice.

4.0 Conclusion

Going electric grants you a lot of options . . . and dozens of decisions. With batteries, this comes down to a few key decisions:

How will you use the battery? Is this is a long term investment, or short term project?
How much do you care about weight?
How much space do you have available?
If considering lithium ion batteries, make sure you pick lithium iron phosphate (LiFePO₄).

These key questions very quickly filter the choices and ease the workload of selecting your batteries. Just take it one stage at a time. And don’t be afraid to change your mind. You got this.

5.0 Acknowledgement

Many thanks to Jeff Cote from Pacific Yacht Systems for helping with much of the background and practical knowledge of these articles. For more information on yacht electrical installations, check out their YouTube channel at: https://www.youtube.com/c/PacificYachtSystems

6.0 References

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